Forming sulfur-based positive electrode active materials

ABSTRACT

In an example of a method for making a sulfur-based positive electrode active material, a carbon layer is formed on a sacrificial nanomaterial. The carbon layer is coated with titanium dioxide to form a titanium dioxide layer. The sacrificial nanomaterial is removed to form a hollow material including a hollow core surrounded by a carbon and titanium dioxide double shell. Sulfur is impregnated into the hollow core.

BACKGROUND

Secondary, or rechargeable, lithium-based batteries are often used inmany stationary and portable devices such as those encountered in theconsumer electronic, automobile, and aerospace industries. The lithiumclass of batteries has gained popularity for various reasons including arelatively high energy density, a general nonappearance of any memoryeffect when compared to other kinds of rechargeable batteries, arelatively low internal resistance, and a low self-discharge rate whennot in use. The ability of lithium ion batteries to undergo repeatedpower cycling over their useful lifetimes makes them an attractive anddependable power source.

SUMMARY

In an example of a method for making a sulfur-based positive electrodeactive material, a carbon layer is formed on a sacrificial nanomaterial.The carbon layer is coated with titanium dioxide to form a titaniumdioxide layer. The sacrificial nanomaterial is removed to form a hollowmaterial including a hollow core surrounded by a carbon and titaniumdioxide double shell. Sulfur is impregnated into the hollow core.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of examples of the present disclosure will become apparent byreference to the following detailed description and drawings, in whichlike reference numerals correspond to similar, though perhaps notidentical, components. For the sake of brevity, reference numerals orfeatures having a previously described function may or may not bedescribed in connection with other drawings in which they appear.

FIGS. 1A through 1D are schematic, cross-sectional views which togetherillustrate an example of a method for making an example of asulfur-based positive electrode active material; and

FIG. 2 is a perspective schematic view of a lithium sulfur batteryshowing a discharging state, the battery including a positive electrodeformed with an example of the sulfur-based positive electrode activematerial disclosed herein.

DETAILED DESCRIPTION

Lithium-based batteries generally operate by reversibly passing lithiumions between a negative electrode (sometimes called an anode) and apositive electrode (sometimes called a cathode). The negative andpositive electrodes are situated on opposite sides of a porous polymerseparator soaked with an electrolyte solution that is suitable forconducting the lithium ions. During charging, lithium ions areinserted/intercalated into the negative electrode, and duringdischarging, lithium ions are extracted from the negative electrode.Each of the electrodes is also associated with respective currentcollectors, which are connected by an interruptible external circuitthat allows an electric current to pass between the negative andpositive electrodes. One example of lithium-based batteries includes thelithium sulfur battery.

The high theoretical capacity (e.g., 1672 mAh/g) of sulfur renders itdesirable for use as a positive electrode material in lithium sulfurbatteries. However, it has been found that positive electrode materialswith high specific capacities also undergo large electromechanical(volume) expansion and contraction during charging/discharging of thebattery. The large volume change (e.g., about 80% for sulfur)experienced by the electrode materials during charging/dischargingcauses the respective material to fracture, decrepitate, or otherwisemechanically degrade, which results in a loss of electrical contact andpoor life cycling.

Moreover, the life cycle of lithium sulfur batteries may be limited bythe migration, diffusion, or shuttling of certain species from thepositive electrode during the battery discharge process, through theporous polymer separator, to the negative electrode.

In lithium sulfur batteries, this species includes S_(x) polysulfidesgenerated at a sulfur-based positive electrode. The S_(x) polysulfidesgenerated at the sulfur-based positive electrode are soluble in theelectrolyte, and can migrate to the negative electrode where they reactwith the negative electrode in a parasitic fashion to generatelower-order polysulfides. These lower-order polysulfides diffuse back tothe positive electrode and regenerate the higher forms of polysulfide.As a result, a shuttle effect takes place. This effect leads todecreased sulfur utilization, self-discharge, poor cycleability, andreduced Coulombic efficiency of the battery. It is believed that even asmall amount of polysulfide at the negative electrode can lead toparasitic loss of active lithium at the negative electrode, whichprevents reversible electrode operation and reduces the useful life ofthe lithium sulfur battery.

In the examples disclosed herein, a cage-like structure is formed and asulfur-based active material is impregnated into the cage-likestructure. This cage-like structure includes a porous, double shell witha layer of carbon and a layer of titanium dioxide (TiO₂). When the TiO₂layer is doped, the double shell can act as an excellent electronicconductor in order to conduct electrons during a battery operation.Additionally, the active material is present in a hollow core that issurrounded by the cage-like structure. The hollow core accommodates thevolumetric expansion and contraction of the active material duringcharging/discharging cycles.

Referring now to FIGS. 1A through 1D, one example of the method formaking the positive electrode active material 10 (shown in FIG. 1D) isdepicted. As shown in FIG. 1A, this example of the method forms a carbonlayer 14 on a sacrificial nanomaterial 12.

The sacrificial nanomaterial 12 may be in the form of nanoparticles,nanorods, nanofibers, or nanowires. The sacrificial nanomaterial 12 mayhave at least one dimension on the nanoscale (e.g., up to 1000 nm), andin some instances, may have at least one dimension up to about 20 μm.The sacrificial nanomaterial 12 may be formed of aluminum oxide or apolymer. Examples of the polymer sacrificial nanomaterial 12 includealuminum alkoxide polymers, titanium alkoxide polymers, zinc alkoxidepolymers, titanium organo nitride, polyester, polyurea, polyimides,poly(vinyl chloride), epoxy resins, or the like.

A carbon layer 14 is then formed on the sacrificial nanomaterial 12. Thecarbon may be deposited using any suitable technique. In one example,the carbon layer 14 is formed using reactive sputtering with graphite asthe target. In another example, the carbon layer 14 is formed bysimultaneously exposing a solid graphite target to a plasma treatmentand an evaporation treatment. The simultaneous plasma and evaporationtreatments may be accomplished using pulsed laser deposition, acombination of cathodic arc deposition and laser arc deposition, acombination of plasma exposure and electron beam (e-beam) exposure, acombination of plasma exposure and laser arc deposition, magnetronsputtering, or plasma enhanced chemical vapor deposition. In an example,the maximum deposition rate ranges from about 48 nm/min to about 100nm/min, which can be achieved with a pulse repetition rate ranging fromabout 1 kHz to about 10 kHz. Still other suitable examples fordepositing the carbon include physical vapor deposition (PVD), electronbeam evaporation, magnetron sputter deposition, chemical vapordeposition (CVD), molecular layer deposition (MLD), atomic layerdeposition (ALD), or a wet chemical process.

The carbon layer 14 is a porous, electrically conductive, continuouscoating formed on the surface of the sacrificial nanomaterial 12. Thecarbon layer 14 may be made up of graphitic carbon, having an sp2/sp3ratio ranging from about 70/30 to about 100/1. In an example, the ratioof sp² carbon to sp³ carbon in the carbon coating is about 74 to about26. Depending upon the deposition process that is used, the ratio of sp²carbon to sp³ carbon may be changed by altering the growth rate, theprecursor (target) that is used, and/or the deposition temperature. Forexample, lowering the deposition temperature to room temperature (e.g.,from about 18° C. to about 22° C.) can result in the formation of aprimarily graphitic carbon layer.

The carbon layer 14 may also be doped with titanium or silicon. In anexample, the dopant makes up less than 20% of the carbon layer 16.Doping the carbon layer 14 may be accomplished using a co-sputteringprocess.

In an example, the carbon layer 14 has thickness ranging from about 5 nmto about 50 nm.

Titanium dioxide is then coated on the carbon layer 14 to form atitanium dioxide (TiO₂) layer 16. The titanium dioxide may be depositedusing any suitable technique. In one example, the TiO₂ layer 16 isformed via plasma enhanced chemical vapor deposition, chemical vapordeposition, molecular layer deposition, atomic layer deposition, or awet chemical process.

The TiO₂ layer 16 is a porous, continuous coating formed on the surfaceof the carbon layer 14. The TiO₂ layer 16 may be tailored to beconductive. For example, the TiO₂ layer 16 may be doped with aconductive additive (such as nitrogen, sulfur, phosphorus, boron,silver, iron and/or vanadium). During deposition of the TiO₂ layer 16,argon and the dopant may be mixed with nitrogen gas. In an example, thedopant makes up less than 20% of the TiO₂ layer 16. The TiO₂ layer 16may undergo a phase transition to render the layer 16 conductive. Forexample, the TiO₂ layer 16 may be exposed to annealing in the presenceof the dopant after it is deposited. For another example, the depositionparameters during TiO₂ deposition may be altered to initiate the phasetransition in the presence of the dopant.

In an example, the TiO₂ layer 16 has a thickness ranging from about 2 nmto about 20 nm.

The TiO₂ layer 16 forms an artificial solid electrolyte interface (SEI)layer at the exterior of the positive electrode active material 10 (seeFIG. 1D). The TiO₂ layer 16 minimizes or inhibits the interfacialreactions between the electrolyte and the sulfur 20 that is impregnatedinto the hollow core 18. This prevents an additional SEI layer fromgrowing on the surface of the nanomaterial 12, or reduces the level atwhich an additional SEI layer grows on the surface of the nanomaterial12.

When a wet chemical process is used to deposit each of the layers 14 and16, a layer-by-layer deposition process or sol-gel deposition processmay be used. In these types of processes, a precursor bath is used toform each layer, and the precursor is changed depending upon the layer14, 16 to be formed. In the precursor bath, the precursor chemisorbs andbonds to the sacrificial nanomaterial 12 or the layer 14 formed thereonto form the layer 14 or 16. Examples of suitable precursors for thecarbon layer 14 include graphite or other carbon nanoparticles ornanofibers in a suitable resin mixture (e.g., polyimide amide). Examplesof suitable precursors for the TiO₂ layer 14 include titaniumisopropoxide, titanium isopropoxide, colloidal titania, titaniumalkoxide Ti(OR)₄, titanic acid and derivatives thereof, Titanium (IV)EDTA, ammonium citraperoxotitanates, organometallic titanium salt, andmixtures thereof.

Referring now to FIG. 1C, the sacrificial nanomaterial 12 is thenremoved to form the hollow core 18 surrounded by the double shell, whichincludes the carbon layer 14 and the TiO₂ layer 16. Both the carbonlayer 14 and the TiO₂ layer 16 are porous, and a solvent that can leachout the sacrificial nanomaterial 12 (while leaving the other layers 14,16 intact) may be used to remove the sacrificial nanomaterial 12. Forexample, an alkali solution may be used to remove the aluminum oxidesacrificial nanomaterial 12. Examples of the alkali solution are alkalimetal oxides, such as 1M NaOH or 1 M KOH. When aluminum oxide isutilized, hydrofluoric acid (HF) is not used. For another example, anorganic solvent may be used to remove the polymer nanomaterial 12. Anyorganic solvent may be selected that will dissolve the polymersacrificial nanomaterial 12 and not affect the other layers 14, 16.Examples of suitable solvents for dissolving examples of the polymersacrificial nanomaterial 12 include benzene, xylene, anisole, orderivatives thereof. In one example, the polymer sacrificialnanomaterial 12 is an epoxy resin that can be removed with acetone,toluene, etc. When the polymer sacrificial nanomaterial 12 is utilized,it may alternatively be removed via heating. Heating causes the polymersacrificial nanomaterial 12 to decompose. In an example, heating togreater than 80° C. is suitable for decomposing some examples of thepolymer sacrificial nanomaterial 12. In one example, the polymersacrificial nanomaterial 12 is poly(vinyl chloride) that can be removedby heating at a temperature >200° C. in an inert atmosphere. Stillfurther, when the polymer sacrificial nanomaterial 12 is utilized, itmay alternatively be removed via plasma etching (e.g., via oxygen, CF₄,or CO).

The amount of space that makes up the hollow core 18 may depend upon thedimensions of the sacrificial nanomaterial 12.

After the hollow core 18 is formed, sulfur 20 may be incorporated intothe hollow core 18 using an impregnation process. This is shown in FIG.1D. An example of the impregnation process involves mixing the doubleshell structure (shown in FIG. 1C) with elemental sulfur 20 at a weightratio ranging from 1:2 to 1:5 to form a mixture. The mixture is thenencapsulated in a tube or a container, which can be pumped down to avacuum below 200 mtorr. The encapsulated mixture is vacuumed and sealed.The sealed mixture is then exposed to a temperature for a predeterminedtime in order to infuse the elemental sulfur 20 into the hollow core 18of the double shell structure. The sealed mixture may be exposed to thetemperature under vacuum conditions. When the sulfur impregnationprocess is accomplished via melt infusing, the temperature ranges fromabout 115° C. to about 165° C. and the predetermined time ranges fromabout 5 hours to about 20 hours under vacuum. In this example, thepredetermined time may range from about 5 hours to about 10 hours. Whenthe sulfur impregnation process is accomplished via vapor infusing, thetemperature ranges from about 444° C. to about 500° C. and thepredetermined time ranges from about 5 hours to about 20 hours undervacuum. In this example, the predetermined time may range from about 5hours to about 10 hours.

The sulfur-based positive electrode active material 10 may be referredto as a yolk-double shell structure, at least in part because at leastsome of the sulfur 20 is like a yolk within the double shell structureof the carbon layer 14 and the TiO₂ layer 16. More particularly, thesulfur 20 may not fill the entire volume of the hollow core 18. As such,it is believed that a void (e.g., free space, unoccupied space) remainsat/near the center of the active material 10. In an example, a maximumamount of the sulfur 20 that is present in the hollow core 18 afterimpregnation is believed to occupy less than 90% of the hollow corevolume. In an example, the weight percentage of sulfur 20 in the activematerial 10 ranges from about 75 wt % to about 85 wt % of the totalactive material weight percent. The portion of the hollow core 18 thatremains as the void provides space to accommodate the volumetricexpansion and contraction of the sulfur 20 during battery cycling. Inaddition to being present within the hollow core 18, the sulfur 22 mayalso be present on the various surfaces of the carbon layer 14 due tothe low contact angle (about 4.3°) or good wettability between graphiteand sulfur.

Additionally, the cage-like structure of the carbon layer 14 and theTiO₂ layer 16 secures the sulfur 20 therein. As such, even if the sulfur20 breaks apart, it is contained within the hollow core 18 by the carbonlayer 14 and the TiO₂ layer 16, which may keep the nanomaterial 12pieces in conductive contact with each other. The cage-like, doubleshell structure of the carbon layer 14 and the TiO₂ layer 16 alsoprotects the sulfur 20, and thus suppresses polysulfide dissolution intothe electrolyte. As such, the cage-like double shell structure of thecarbon layer 14 and the TiO₂ layer 16 can mitigate the shuttle effect,and in turn improve the efficiency and life cycle of the lithium sulfurbattery.

It is to be understood that the positive electrode active material 10disclosed herein may be utilized as the active material in a positiveelectrode for a lithium sulfur battery. The positive electrode activematerial 10 may be combined with an additional conductive filler and/ora binder to form the positive electrode.

Since the positive electrode active material 10 includes the conductivecarbon layer 14, the additional conductive filler may or may not beincluded when forming the positive electrode. When included, theconductive filler may be a conductive carbon material. The conductivecarbon material may be a high surface area carbon, such as acetyleneblack (e.g., SUPER P® conductive carbon black from TIMCAL). Theconductive filler may be included to enhance electron conduction betweenthe positive electrode active material 10 and a a positive-side currentcollector.

The binder may be used to structurally hold the positive electrodeactive material 10 together within the positive electrode. Examples ofthe binder include polyvinylidene fluoride (PVdF), polyethylene oxide(PEO), an ethylene propylene diene monomer (EPDM) rubber, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), styrene-butadienerubber carboxymethyl cellulose (SBR-CMC), polyacrylic acid (PAA),cross-linked polyacrylic acid-polyethylenimine, polyimide, or any othersuitable binder material. Other suitable binders include polyvinylalcohol (PVA), sodium alginate, or other water-soluble binders.

In an example of the method for making the positive electrode with thesulfur-based electrode active materials 10, the sulfur-based electrodeactive material 10 may be mixed with the conductive fillers and thebinder(s).

The respective components may be manually mixed by dry-grinding. Afterall these components are ground together, the ground components arecombined with water or organic solvent (depending on the binder used) toform the dispersion/mixture. In an example, the solvent is a polaraprotic solvent. Examples of suitable polar aprotic solvents includedimethylacetamide (DMAC), N-methyl-2-pyrrolidone (NMP),dimethylformamide (DMF), dimethylsulfoxide (DMSO), or another Lewisbase, or combinations thereof.

The dispersion/mixture may be mixed by milling. Milling aids intransforming the dispersion/mixture into a coatable slurry. Low-shearmilling or high-shear milling may be used to mix the dispersion/mixture.The dispersion/mixture milling time ranges from about 10 minutes toabout 20 hours depending on the milling shear rate. In an example, arotator mixer is used for about 20 minutes at about 2000 rpm to mill thedispersion/mixture.

In an example of the dispersion/mixture for the positive electrode, theamount of the sulfur-based positive electrode active materials 10 rangesfrom about 70 wt. % to about 95 wt. % (based on total solid wt. % of thedispersion/mixture), the amount of the conductive filler ranges from 0wt. % to about 15 wt. % (based on total solid wt. % of thedispersion/mixture), and the amount of the binder ranges from about 5wt. % to about 15 wt. % (based on total solid wt. % of thedispersion/mixture).

The slurry is then coated or deposited onto the respective currentcollector (e.g., copper for the negative electrode and aluminum for thepositive electrode). The slurry may be deposited using any suitabletechnique. As examples, the slurry may be cast on the surface of thecurrent collector, or may be spread on the surface of the currentcollector, or may be coated on the surface of the current collectorusing a slot die coater.

The deposited slurry may be exposed to a drying process in order toremove any remaining solvent and/or water. Drying may be accomplishedusing any suitable technique. For example, drying may be performed at anelevated temperature ranging from about 60° C. to about 130° C. In someexamples, vacuum may also be used to accelerate the drying process. Asone example of the drying process, the deposited slurry may be exposedto vacuum at about 120° C. for about 12 to 24 hours. The drying processresults in the formation of the positive electrode.

An example of the positive electrode 24 formed with the positiveelectrode active material 10 is depicted in FIG. 2, which also shows anexample of the lithium sulfur battery 22.

In the lithium sulfur battery 22, the positive electrode 24 on thepositive-side current collector 26 is paired with a negative electrode28 on a negative side current collector 30. The current collectors 26,30 collect and move free electrons to and from an external circuit 36.The positive-side current collector 26 may be formed from aluminum orany other appropriate electrically conductive material known to skilledartisans. The negative-side current collector 30 may be formed fromcopper or any other appropriate electrically conductive material knownto skilled artisans.

The negative electrode 28 of the lithium sulfur battery 22 may includeany lithium host material that can sufficiently undergo lithiation anddelithiation with copper functioning as the negative terminal/currentcollector 30 of the lithium sulfur battery 22. Examples of suitablenegative electrode active materials include graphite, lithium titanate,lithiated silicon (e.g., LiSi_(x)), lithiated tin, or lithium foil.Graphite may be desirable for the negative electrode 28 because itexhibits reversible lithium intercalation and deintercalationcharacteristics, is relatively non-reactive, and can store lithium inquantities that produce a relatively high energy density. Commercialforms of graphite that may be used to fabricate the negative electrode28 are available from, for example, Timcal Graphite & Carbon (Bodio,Switzerland), Lonza Group (Basel, Switzerland), or Superior Graphite(Chicago, Ill.).

The negative electrode 28 may also include any of the conductive fillersand binders previously described.

The lithium sulfur battery 22 also includes the porous polymer separator32 positioned between the positive and negative electrodes 24, 28. Theporous polymer separator 32 may be formed, e.g., from a polyolefin. Thepolyolefin may be a homopolymer (derived from a single monomerconstituent) or a heteropolymer (derived from more than one monomerconstituent), and may be either linear or branched. If a heteropolymerderived from two monomer constituents is employed, the polyolefin mayassume any copolymer chain arrangement including those of a blockcopolymer or a random copolymer. The same holds true if the polyolefinis a heteropolymer derived from more than two monomer constituents. Asexamples, the polyolefin may be polyethylene (PE), polypropylene (PP), ablend of PE and PP, or multi-layered structured porous films of PEand/or PP. Commercially available porous separators 22 include singlelayer polypropylene membranes, such as CELGARD 2400 and CELGARD 2500from Celgard, LLC (Charlotte, N.C.). It is to be understood that theporous separator 36 may be coated or treated, or uncoated or untreated.For example, the porous separator 36 may or may not be coated or includeany surfactant treatment thereon.

In other examples, the porous separator 32 may be formed from anotherpolymer chosen from polyethylene terephthalate (PET), polyvinylidenefluoride (PVdF), polyamides (Nylons), polyurethanes, polycarbonates,polyesters, polyetheretherketones (PEEK), polyethersulfones (PES),polyimides (PI), polyamide-imides, polyethers, polyoxymethylene (e.g.,acetal), polybutylene terephthalate, polyethylenenaphthenate,polybutene, polyolefin copolymers, acrylonitrile-butadiene styrenecopolymers (ABS), polystyrene copolymers, polymethylmethacrylate (PMMA),polyvinyl chloride (PVC), polysiloxane polymers (such aspolydimethylsiloxane (PDMS)), polybenzimidazole (PBI), polybenzoxazole(PBO), polyphenylenes (e.g., PARMAX™ (Mississippi Polymer Technologies,Inc., Bay Saint Louis, Miss.)), polyarylene ether ketones,polyperfluorocyclobutanes, polytetrafluoroethylene (PTFE),polyvinylidene fluoride copolymers and terpolymers, polyvinylidenechloride, polyvinylfluoride, liquid crystalline polymers (e.g., VECTRAN™(Hoechst AG, Germany) and ZENITE® (DuPont, Wilmington, Del.)),polyaramides, polyphenylene oxide, and/or combinations thereof. It isbelieved that another example of a liquid crystalline polymer that maybe used for the porous separator 32 is poly(p-hydroxybenzoic acid). Inyet another example, the porous separator 32 may be chosen from acombination of the polyolefin (such as PE and/or PP) and one or more ofthe other polymers listed above.

The porous separator 32 may be a single layer or may be a multi-layer(e.g., bilayer, trilayer, etc.) laminate fabricated from either a dry orwet process.

In the lithium sulfur battery 22, the porous polymer separator 32operates as both an electrical insulator and a mechanical support. Theporous polymer separator 32 is sandwiched between the positive electrode24 and the negative electrode 28 to prevent physical contact between thetwo electrodes 24, 28 and the occurrence of a short circuit. The porouspolymer separator 32, in addition to providing a physical barrierbetween the two electrodes 24, 28 ensures passage of lithium ions(identified by the Li⁺) and some related anions through an electrolytesolution 34 filling its pores.

Any appropriate electrolyte solution 34 that can conduct lithium ionsbetween the negative electrode 28 and the positive electrode 24 may beused in the lithium sulfur battery 22. In one example, the non-aqueouselectrolyte solution may include lithium salt(s) dissolved in an etherbased organic solvent. The ether based organic solvent may be composedof cyclic ethers, such as 1,3-dioxolane, tetrahydrofuran,2-methyltetrahydrofuran, and chain structure ethers, such as1,2-dimethoxyethane, 1-2-diethoxyethane, ethoxymethoxyethane,tetraethylene glycol dimethyl ether (TEGDME), polyethylene glycoldimethyl ether (PEGDME), and mixtures thereof. Examples of the lithiumsalt include LiClO₄, LiAlCl₄, LiI, LiBr, LiSCN, LiBF₄, LiB(C₆H₅)₄,LiAsF₆, LiCF₃SO₃, LiN(FSO₂)₂ (LIFSI), LiN(CF₃SO₂)₂ (LITFSI), LiPF₆,LiB(C₂O₄)₂ (LiBOB), LiBF₂(C₂O₄) (LiODFB), LiPF₃(C₂F₅)₃ (LiFAP),LiPF₄(CF₃)₂, LiPF₄(C₂O₄) (LiFOP), LiNO₃, LiPF₃(CF₃)₃, LiSO₃CF₃, andmixtures thereof. In an example, the concentration of the salt in theelectrolyte solution 24 is about 1 mol/L. LiNO₃ may also be added to theelectrolyte solution 34 as another additive, in addition to anotherlithium salt. In these instances, the concentration of the lithium saltmay be about 0.6 mol/L plus the LiNO₃ additive.

The lithium sulfur battery 22 also includes the interruptible externalcircuit 36 that connects the positive electrode 24 and the negativeelectrode 28. The lithium sulfur battery 22 may also support a loaddevice 38 that can be operatively connected to the external circuit 36.The load device 38 may be powered fully or partially by the electriccurrent passing through the external circuit 36 when the lithium sulfurbattery 22 is discharging. While the load device 38 may be any number ofknown electrically-powered devices, a few specific examples of apower-consuming load device include an electric motor for a hybridvehicle or an all-electrical vehicle, a laptop computer, a cellularphone, and a cordless power tool. The load device 38 may also, however,be a power-generating apparatus that charges the lithium sulfur battery22 for purposes of storing energy. For instance, the tendency ofwindmills and solar panels to variably and/or intermittently generateelectricity often results in a need to store surplus energy for lateruse.

The lithium sulfur battery 22 can include a wide range of othercomponents that, while not depicted here, are nonetheless known toskilled artisans. For instance, the lithium sulfur battery 22 mayinclude a casing, gaskets, terminals, tabs, and any other desirablecomponents or materials that may be situated between or around thepositive electrode 24 and the negative electrode 28 forperformance-related or other practical purposes. Moreover, the size andshape of the lithium sulfur battery 22, as well as the design andchemical make-up of its main components, may vary depending on theparticular application for which it is designed. Battery-poweredautomobiles and hand-held consumer electronic devices, for example, aretwo instances where the lithium sulfur battery 22 would most likely bedesigned to different size, capacity, and power-output specifications.The lithium sulfur battery 22 may also be connected in series and/or inparallel with other similar lithium sulfur batteries 22 to produce agreater voltage output and current (if arranged in parallel) or voltage(if arranged in series) if the load device 38 so requires.

The lithium sulfur battery 22 can generate a useful electric currentduring battery discharge (shown in FIG. 2). During discharge, thechemical processes in the battery 22 include lithium (Li⁺) dissolutionfrom the negative electrode 28 and incorporation of the lithium cationsinto sulfur or high form polysulfide anions (i.e., S_(x) ²⁻) within thesulfur-based positive electrode active materials 10. As such,polysulfides are formed (sulfur is reduced) within the active materials10 in the positive electrode 24 in sequence while the battery 22 isdischarging. The chemical potential difference between the positiveelectrode 24 and the negative electrode 28 (ranging from approximately1.5V to 3.0V, depending on the exact chemical make-up of the electrodes24, 28) drives electrons produced by the dissolution of lithium at thenegative electrode 28 through the external circuit 36 towards thepositive electrode 24. The resulting electric current passing throughthe external circuit 36 can be harnessed and directed through the loaddevice 38 until the level of intercalated lithium in the negativeelectrode 28 falls below a workable level or the need for electricalenergy ceases.

The lithium sulfur battery 22 can be charged or re-powered at any timeby applying an external power source to the lithium sulfur battery 44 toreverse the electrochemical reactions that occur during batterydischarge. During charging (not shown), lithium plating to the negativeelectrode 28 takes place and sulfur formation at the positive electrode24 takes place. The connection of an external power source to thelithium sulfur battery 22 compels the otherwise non-spontaneousoxidation of lithium at the positive electrode 24 to produce electronsand lithium ions. The electrons, which flow back towards the negativeelectrode 28 through the external circuit 36, and the lithium ions(Li⁺), which are carried by the electrolyte 34 across the porous polymerseparator 32 back towards the negative electrode 28, reunite at thenegative electrode 28 and replenish it with lithium for consumptionduring the next battery discharge cycle. The external power source thatmay be used to charge the lithium sulfur battery 22 may vary dependingon the size, construction, and particular end-use of the lithium sulfurbattery 22. Some suitable external power sources include a batterycharger plugged into an AC wall outlet and a motor vehicle alternator.

It is to be understood that the ranges provided herein include thestated range and any value or sub-range within the stated range. Forexample, a range of from about 5 nm to about 50 nm should be interpretedto include not only the explicitly recited limits of from about 5 nm toabout 50 nm, but also to include individual values, such as 12 nm, 25.5nm, etc., and sub-ranges, such as from about 20 nm to about 40 nm, etc.Furthermore, when “about” is utilized to describe a value, this is meantto encompass minor variations (up to +/−10%) from the stated value.

Reference throughout the specification to “one example”, “anotherexample”, “an example”, and so forth, means that a particular element(e.g., feature, structure, and/or characteristic) described inconnection with the example is included in at least one exampledescribed herein, and may or may not be present in other examples. Inaddition, it is to be understood that the described elements for anyexample may be combined in any suitable manner in the various examplesunless the context clearly dictates otherwise.

In describing and claiming the examples disclosed herein, the singularforms “a”, “an”, and “the” include plural referents unless the contextclearly dictates otherwise.

While several examples have been described in detail, it is to beunderstood that the disclosed examples may be modified. Therefore, theforegoing description is to be considered non-limiting.

The invention claimed is:
 1. A method for making a sulfur-based positiveelectrode active material, the method comprising: forming a carbon layeron a sacrificial nanomaterial; coating the carbon layer with titaniumdioxide to form a titanium dioxide layer; rendering the titanium dioxideconductive by doping or through a phase transition; removing thesacrificial nanomaterial, thereby forming a hollow material including ahollow core surrounded by a carbon and titanium dioxide double shell;and impregnating sulfur into the hollow core.
 2. The method as definedin claim 1 wherein the sacrificial nanomaterial is formed of aluminumoxide and wherein the removing of the sacrificial nanomaterial isaccomplished using an alkali solution.
 3. The method as defined in claim1 wherein the impregnating of the sulfur into the hollow core includes:mixing the hollow material with elemental sulfur at a weight ratioranging from 1:2 to 1:5, thereby forming a mixture; encapsulating themixture; vacuuming and sealing the mixture; and exposing the sealedmixture to a temperature for a predetermined time, thereby infusing theelemental sulfur into the hollow core of the hollow material undervacuum conditions to impregnate the hollow core with sulfur.
 4. Themethod as defined in claim 3 wherein: infusing the elemental sulfur intothe hollow core to impregnate the hollow core with sulfur isaccomplished via melt infusing; the temperature ranges from about 115°C. to about 165° C.; and the predetermined time ranges from about 5hours to about 10 hours under vacuum.
 5. The method as defined in claim3 wherein: infusing the elemental sulfur is accomplished via vaporinfusing; the temperature ranges from about 444° C. to about 500° C.;and the predetermined time ranges from about 5 hours to about 10 hoursunder vacuum.
 6. The method as defined in claim 1 wherein thesacrificial nanomaterial is selected from the group consisting ofaluminum oxide nanoparticles, aluminum oxide nanorods, aluminum oxidefibers, and aluminum oxide nanowires.
 7. The method as defined in claim1 wherein the forming of the carbon layer and the coating of the carbonlayer with the titanium dioxide are accomplished by plasma-enhancedchemical vapor deposition, chemical vapor deposition, molecular layerdeposition, atomic layer deposition, or a wet chemical process.
 8. Themethod as defined in claim 1 wherein the forming of the carbon layer isaccomplished by one of: a deposition technique that involves reducing adeposition temperature down to about 18° C. to about 22° C.; orsputtering with graphite as a target.
 9. The method as defined in claim8, further comprising doping the carbon layer with titanium or silicon.10. The method as defined in claim 1, wherein the titanium dioxide isdoped with a conductive additive selected from the group consisting of:nitrogen, sulfur, phosphorus, boron, silver, iron, vanadium, andcombinations thereof, and wherein the dopant comprises more than 0% andless than 20% of the titanium dioxide layer.
 11. The method as definedin claim 1 wherein the carbon layer has a thickness ranging from about 5nm to about 50 nm and a sp²/sp³ ratio ranging from about 70/30 to about100/1; and wherein the titanium dioxide layer has a thickness rangingfrom about 2 nm to about 20 nm.
 12. The method as defined in claim 1wherein the carbon layer and the titanium dioxide layer form a porousdouble shell.
 13. The method as defined in claim 1 wherein afterimpregnation of sulfur into the hollow core less than about 90% of thehollow core is occupied by sulfur.
 14. A method for making asulfur-based positive electrode active material, the method comprising:forming a carbon layer on a sacrificial nanomaterial; coating the carbonlayer with titanium dioxide to form a titanium dioxide layer, whereinthe forming of the carbon layer and the coating of the carbon layer withthe titanium dioxide are accomplished by plasma-enhanced chemical vapordeposition, chemical vapor deposition, molecular layer deposition,atomic layer deposition, or a wet chemical process; removing thesacrificial nanomaterial, thereby forming a hollow material including ahollow core surrounded by a carbon and titanium dioxide double shell;and impregnating sulfur into the hollow core.
 15. The method as definedin claim 14 wherein after impregnation of sulfur into the hollow coreless than about 90% of the hollow core is occupied by sulfur.
 16. Themethod as defined in claim 14, further comprising doping the carbonlayer with titanium or silicon, wherein the carbon layer comprises morethan 0% to less than 20% of the dopant.
 17. The method as defined inclaim 14, further comprising rendering the titanium dioxide conductiveby doping or through a phase transition.